As an image display device substitutable for liquid crystal display (LCD), image display devices with the use of technology such as an electrophoresis method, an electro-chromic method, a thermal method, dichroic-particles-rotary method are proposed.
As for these image display device, it is conceivable as inexpensive visual display device of the next generation from a merit having wide field of vision close to normal printed matter, having smaller consumption with LCD, spreading out to a display for portable device, and an electronic paper is expected.
Recently, electrophoresis method is proposed that microencapsulate dispersion liquid made up with dispersion particles and coloration solution and dispose the liquid between faced substrates. However, in the electrophoresis method, there is a problem that a response speed is slow by the reason of viscosity resistance because the particles migrate among the electrophoresis solution. Further, there is a problem of lacking imaging repetition stability, because particles with high specific gravity of titanium oxide is scattered within solution of low specific gravity, it is easy to subside, difficult to maintain a stability of dispersion state. Even in the case of microencapsulating, cell size is diminished to a microcapsule level in order to make it hard to appear, however, an essential problem was not overcome at all.
Besides the electrophoresis method using behavior in the solution, recently, a device without using the solution is proposed, in which two or more groups of particles having different colors and different charge characteristics are sealed between two substrates and the particles, to which an electrostatic field is applied, are made to fly and move so as to display an image. [The Imaging Society of Japan “Japan Hardcopy '99” (Jul. 21-23, 1999) Transaction Pages 249-252] Since this device is a dry type as opposed to the electrophoresis method, there is a merit such that a moving resistance of the particles is small and thus a response speed is fast.
The dry-type display device mentioned above has an operation mechanism such that a mixture of two kinds of the particles having different colors and different charge characteristics is sandwiched by electrode plates and an electric field is generated between the electrode plates by applying a voltage to the electrode plates, thereby flying the charged particles having different charge characteristics in a different direction to obtain a display element.
As to forces applied to the particles, there are an attraction force between the particles due to Coulomb' force, an imaging force with respect to the electrode panel, an intermolecular force, a liquid bridging force and a gravity.
When the force applying to the particles due to the electric field is larger than total forces mentioned above, the particle fly occurs.
In the case that an actual drive circuit in the image display device takes into consideration, it is preferred that the drive voltage is lowered. As a large factor for defining the drive voltage, there are mainly an intermolecular force and a liquid bridging force. If these factors are made to be lowered, a decrease of the drive voltage is instantly achieved. Therefore, it is very important to improve these characteristics.
Moreover, in order to obtain god operation properties, it is preferred that an electrostatic property is made to be higher to some extent. Therefore, it is another task to obtain the particles having an excellent electrostatic property. The electrostatic property of the particle itself for the image display is the most important factor when a force generated by applied electric field and an adhesion force between the particles or to the substrate are controlled. However, the electrostatic property of the particle is normally under control of a material of the particle itself, and thus it is difficult to control it accurately by the particle itself.
Further, in the case that fine particles are used in a display element, it is necessary to use white color particles and black color particles so as to make a contrast on color tone clear. Contrary to this, polymerized fine particles obtained from a general-purpose resin show an achromatic color, but, since they are fine particles, they look like a white color by an irregular reflection of light. However, in the case that they are used with the black particles in the image display device, a contrast ratio lacks and thus whiteness is decreased.
In order to improve the whiteness, there is a method such that white fine powders such as titanium oxide powders or zinc oxide powders are included in the particle. However, in the case that the white fine particles are to be included during a particle polymerization by means of a polymerized method, it is necessary to control a surface affinity by performing a coupling agent treatment with respect to the white fine powders, so that the white fine powders are included in the particle effectively. Therefore, the producing method becomes very complicated.
Further, in the case that the method mentioned above is utilized, it is normally difficult to control a particle size. For example, a method, such that the particles are produced by kneading, grinding and classifying a mixture of a main resin and white fine powders constituting the particle, is proposed. However, in this case, it is not possible to obtain the particles having a narrow particle size distribution if the classifying operation is not performed. Moreover, since a mechanical grinding is performed so as to obtain the fine particles, it is difficult to obtain the particles having a particle size of 8 μm or less even if the fine particles having a particle size under the above level are to be obtained.